A low-profile antenna with a vertically polarized fan beam (approximately 2.degree..times.50.degree.) is needed for C-band Aircraft Interferometric SAR (Synthetic Aperture Radar) applications. The main beam of the antenna is required to be fixed at the broadside direction. The available physical area for the antenna is 1.7 m by 0.17 m. To conformably mount the antenna outside the aircraft's surface, microstrip array structure with thin substrate material is ideal for the application.
The simplest form of feed system for such a relatively long microstrip array is series feeding in which not only the dielectric insertion loss of the feed transmission lines is minimized but the leakage radiations from the lines are also reduced when compared to a complete corporate feed system. In addition, the space usage of the given aperture is significantly improved in a series fed array structure.
There are two types of series feeding techniques: resonant feed and traveling wave feed. In a resonant feed array, no impedance matching to the elements is necessary and the resulting multiply bounced waves in the transmission line will radiate into space through the elements with phases equal to the primary radiated waves due to proper element spacing. However, because of the multiple bounces, the insertion loss that occurs in the transmission line of a resonant feed array is generally higher than that in a traveling wave feed array. In addition, because of the phase coherence requirement of the multiply bounced waves, the resonant array has extremely narrow bandwidth. The traveling wave feed technique is therefore preferred, but that type of series feeding has its own problems of beam squint and insertion loss. This is because the main beam angle of an array series fed from one end will be very sensitive to frequency change due to the progressive phase change of the series fed elements.